Television system



July 14, 1,942,

R. D. KELL TELEVISION SYSTEM Alum 1i 3 E 55 A,

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UNITED STATES PATENT OFFICE TELEVISION `SYSTlllVil Ray Davis Ken, Haddonieigh, N. J., assignmto Radio Corporation of America, a corporation of Delaware Application September 25, 1931, Serial No. 565,226

4 Claims.

This invention relates to improvements in television systems. A

In the operation of television systems, it is important that the background brilliancy of the reproduced picture shall vary in accordance with the brilliancy of the scene being transmitted.v For this purpose it has been necessary, heretofore, to continually adjust the apparatus manually.

One of the objects of my invention is to provide an improved television system wherein the degree of background brilliancy of the reproduced picture is faithfully varied automatically in accordance with variations in the intensity of the received picture signals.

Other objects and advantages will hereinafter appear. l

In accordance with my invention, use is made of the fact that the background component ofva Figs. 9 and 10 are fragmentary views similarv to Fig. 8, illustrating modifications.

The system shown in Fig. 1 is of the same general type as that described in detail in the copending application of Arthur W. Vance, Serial No. 544,959, filed June 17, 1931, now Patent 2,137,039, issued November 15, 1938. A suitable'l radio kreceiver I supplies picture signals to a resistor II from which these signals are applied to the control grid I2 of a cathode ray tube I3. For the purpose of causing the cathode ray M to scan the uorescent screen I5, a generator I6 picture bears a definite relation to the amplitude of the' picture signals developed during scanning action at the transmitting station. 'I The brilliancy of the picture background is faithfully reproduced at the receiving station by making the' direct current or background component of the picture signals proportional to the area of the same in the white'direction.

More particularly, in accordance with my invention, the received picture signals are applied to the grid of a condenser grid-leak detector to cause the voltage in the plate circuit to vary inversely proportionally to the area of the picture signals in the white direction. A background component of lthe picture is then re-introduced Into Ithe control circuit of the receiving apparatus such, for example, as the grid circuit of a cathode ray tube or a control circuit for a neon tube, by applying the plate voltage of the detector to the control circuit of the receiving apparatus.

For the purpose of illustrating my invention, an embodiment thereof is shown inthe ldrawings, wherein Figure 1 is adiagrammatic view of a television receiving system embodying my invention;

Figs. 2a, 2b, 2c, and 3 are graphical views illus- .trative of the principle of operation;

Fig. 4 is a fragmentary view similar to Fig. 1, illustrating a modification of my invention;

Fig. 5 is a diagrammatic view of a television transmitter which may be vemployed in a television system embodying my invention;

Fig. 6 is avside elevational view of the scanning disc used at the transmitting station, lookx ing toward the left in Fig. 5;

Figs. '7 and 8 are enlarged fragmentary views looking toward the left in. Fig. 5; and

causes a 1200 cycle saw-tooth current-wave to pass through thehorizontal deecting coils Il vwhile av generator I8 causes la 20 cycle saw-tooth current wave to pass through the vertical deecting coils I9.

For the purpose of synchronizing the scanning action at the receiver with such action at' the transmitter, synchronizing impulses are received at the end of each scanning-line period, 1200 of these impulses being received during each picture-frame period. For the purpose of framing, a single synchronizing impulse, received at the end of each picture-frame period, is substantially greater in amplitude than the others. The adjustment of the generator I6 is such that the same is not responsive to the peak picture signals, but is responsive to the synchronizing impulses 20 of substantially greater amplitude, as indicated in Fig. 2c. The adjustment of the generator I8 is such that the same is responsive only to the single framing impulse which is substantially greater in amplitudeA than the peak picture signals, and the other synchronizing impulses 20.'

The cathode ray tube I3 is supplied with the usual operating potentials, as indicated.

For the purpose of automatically controlling the background brilliancy of the picture, the grid circuit of a suitable condenser grid-leak detector 2| is also supplied with the picture signals from the resistance II, as shown. 'Ihe'plate voltage of the detector 2| is applied to the control grid I2 of the cathode ray tube' through a connection 22 including a suitable resistor 23.

The' operation is as follows:

"i When there is no picture, the only signals reyceived are the synchronizing inpulses -20, as

shown in Fig. 2a. When these impulses are applied to the grid circuit of the detector 2|, there is little or no decrease in the plate current of the detector, as show n by the vcharacteristic curve of the detector in Fig. 3.

When there is a little white in the picture, for

example, White letters on a black background,

the received signals are as shown Vin Fig. 2b. When these signals are applied to the grid circuit of the detector 2|, as shown at b in Fig. 3, there is a corresponding slight decrease in the plate current of the detector. p

When vthere is a great deal of white in the picture, the received signals are as shown in Fig. 2c. When these signals are applied to the grid circuit of the detector 2|, as shown at c in Fig. 3', there is a corresponding large'decrease in the plate current of the detector. l

The bias on the control grid I2 and the adjustment of thedetector 2| are such that with no picture signal on the grid of the detector, the screen l is dark, that is, the cathode ray I4 is cut off. I

Upon an increase in the picture signal intensity in the White direction, the amount of space current flowing through the detector 2i will decrease correspondingly in the Well known manner. The plate voltage of the detector 2|, therefore, increases correspondingly and, since this voltage constitutes a bias on the control grid I2 of the cathode ray tube, the negative bias on` this tube will decrease to make the screen l5 correspondingly brighter.

From the foregoing, it will be seen that upon an increase in picture signal strength in the positive or white direction, the detector 2| causes a corresponding decrease in bias on the cathode ray tube, thereby causing the screen I5 to be more brilliantly illuminated. Upon a decrease in picture signal strength, the detector` 2| causes an increase in bias on the tube I3, thereby causing the screen I5 t0 be illuminated less brilliantly. When there are no picture' signals, the adjustment, as stated, is such that the ray I4 is cut off to make the screen I5 dark.

My invention may be embodied in any type of television receiving apparatus by reintroducing into the control circuit thereof the background component of the picture as provided by the detectorf 2|.,. For example, Yif the receiver utilizes anejon tube25, as shown in Fig. 4,- the plate circuitjofjthe detector 2| is connected to the grid circuitvofga suitable' amplifier 26 to the plate circuitfof.; which the tube 25 is connected. The action, then, is the same as that described in connection with Fig. 1, the overall brilliancyof the tube 25 varying directly proportionally to variations in the intensity of the received picture signals.

It will be ,understood that a transmitter of the same general type as that described in the above mentioned Vance application may be employed for transmitting signals of the character illustrated in Figs. 2a, 2b and 2c. 'The ltransmitter described and claimed inthe said Vance appliand a lens or lens system 33. A mask 34 is sup,

ported between the light source and disc 30.

The disc 30 is provided, for example, with sixty scanning holes A1-Aso, spirally arranged in the usual manner. The holes Afm, A1, and Az are plugged. as indicated. The reason for this will be hereinafter explained.

Light reflected from an object 35 excites a photoelectric cell 36,. ora bank of such cells, to develop picture signals which are amplified by an amplifier 3l and applied to the input line 38 of a second amplifier 39. The` second amplifier 39 is connected, as shown, to a suitable-modulator 40 which controls a radio transmitter 4|.

For the purpose of synchronizing operations at the transmitting and receiving stations, electrical synchronizing impulses, at the horizontal synchronizing and framing frequencies, are required. The present improved methods and means for developing these two frequencies will now be explained, with reference more particularly to Figs. 6 to 10.

The disc 30 `is provided at the outer edge thereof with concentrically arranged holes a1 to aso, through which light from a light source 42 passes to a photoelectric cell 43, as shown in Fig. 5. A screen 44, provided with an aperture 45 substantially equal in Width to that of the synchronizing holes, is interposed between the disc 39 and the light source 42. A lens 46 may be employed to focus the light on the photoelectric cell'.

There being sixty synchronizing holes, the photoelectric cell 43 develops electrical impulses at a horizontal synchronizing frequency of r1200 cycles per second. The impulses or signals at this frequency are amplified by a suitable amplifier 4l and are supplied directly to the input line 38 of the amplifier 39, in parallel with the amplied picture signals.

The width of the opening 48 in the mask 34 is made less than the circumferential distance between adjacent scanning holes, as shown in Fig. 7, to provide for periods during which no picture signals are transmitted. The adjustment of the screen 44 is such that during thesel periods the synchronizing holes ai -to aan move, across the aperture 45.

Three picture lines are omitted by plugging synchronizing impulses because the width of` hole a1 is equal to the Width of the other synchronizing holes. The amplitude of the framing impulses, however, is substantially greater because of the greater length of hole a1.

From the foregoing, it will be seen that although the synchronizing signals are transmitted in the same channel with the picture signals, the former are developed only during periods when the picture signals are cut ofi, and for this reason interfere in no way with the picture fidelity at the receiving station. I

When it is vdesired that the framing impulses be of greater duration as well as be greater in amplitude than the other synchronizing impulses, the hole a1 in Figs. 6 and 8 is given the shape shown in Fig. 9. That is, the size and shape of the framing hole in Fig.v 9 is such that the same not only moves across aperture 45 for a longer period, but during such movement admits more light to the photoelectric cell 43 than the other synchronizing holes.

If the requirements are such that the framing impulses be of the same amplitude but of greater duration than the other synchronizing impulses, in order to distinguish the former from the latter is, 'the size and shape of the framing hole in Fig. Y

10 is such that the saine moves across aperture 45 for a lo'nger period, and during such movement admits the same maximum'amount of light to cell 43 as the other synchronizing holes. t

The number of stages in amplifiers 31 and 4l is such that the output voltage of amplifier 4l is 180 degrees out of phase with that of amplifier 31, an increase in light intensity on the picture cell 36 causing a corresponding increase, in theA positive sense, in the output voltage of amplifier 31. Therefore, when the synchronizing cell 43 is exposed to the light from source d2, there is a sharp increase, in the negative direction, in the output voltage of amplifier 67.

The synchronizing holes a1 to asn may be disposed on the inside of the scanning holes, if desired. It is also contemplated, if and when found desirable, to employ a separate disc for synchronizing purposes. In such case, the synchronizing disc would bedriven by motor 3|, and provided with synchronizing holes arranged and formed as described above.

For convenience of explanation, the scanning holes Aso, A1 and A2 have been described as such,

and it has been stated that these holes are plugged to omit several picture lines. In actual practice, however, the disc is not apertured at all at these spots.

While the scanning disc has been described as providing for 60 picture lines, it will be understood that the design may be varied to provide for more or less lines, as might be found desirable for particular conditions. Furthermore, the scanning disc may be replaced by a scanning cylinder or other equivalent rotating means of well known construction. Ifv a scanning cylinder is used, the same may be provided with the synchronizing holes, or a separate cylinder may be employed for the synchronizing action, as explained above in connection with the disc construction.

The ampliers 31, 39 and 4U, and the usual power amplifier forming part of the radio transmitter, are of any well known construction havlng the following desirable characteristics:

(1) Amplification substantially constant at all required frequencies.

(2) The time-lag in the amplifiers to be either zero or substantially constant at all required frequencies.

(3) The amplification to be substantially constant at all required signal amplitudes. The modulator to give practically constant modulating power over the required frequency range.

I claim as my invention:

1. In the art of television wherein received picture signals are applied to an electron beam in- 'tensity control electrode of a cathode ray tube to eiect reproduction of la view, the method of operation which comprises applying said signals directly to said control electrode, rectifying said signals to produce a -control voltage proportional to their area in the white direction and applying said control voltage to said control electrode.

2. In a television system, means comprising a cathode ray tube for reproducing a picture, an electron beam intensity control circuit for said tube, means connected to said circuit for supplying the same with picture signals, and a thermionic detector having its input circuit supplied with said signals to rectify them in accordance with their area on the white side of the alternating current axis and having its output circuit connected to said control circuit independently of the connection between said signal-supply means and said control circuit.

3. The invention according to claim 2 wherein the detector is of the grid-leak biased type.

4. A television receiver comprising acathode ray tube having an electron beam intensity control electrode, means for applying picture signal over one path to said electrode for reproducing a view, means positioned in another path for rectifying said picture signal to produce a control voltage that varies in accordance with the area of said signal on the white side of its alternating an increase in said area.

current axis, and means for applying said control voltage to said control electrode in such sense as to make it more positive in response to RAY DAVIS KELL. 

